The invention relates to endoscopic instruments. More particularly, the invention relates to an endoscopic scissors instrument with a cammed biasing surface between scissor blades whereby the cutting edges of the scissor blades are variably biased together as the scissor blades pivot with respect to each other.
Various kinds of scissors are well known in the art. Scissors generally consist of a pair of scissor blades each having an internal cutting edge and a pivot means whereby at least one of the scissor blades pivotally engages the other scissor blade such that its inside cutting edge engages the inside cutting edge of the other scissor blade in bearing contact from a point near the pivot means to a point along the cutting edges distal from the pivot means. In order to effect a smooth cutting action, the engaging cutting edges must be kept in a single moving point of close contact throughout the pivoting of the blades. Continuous close contact of the cutting edges is usually accomplished two ways: first, by biasing the blades together at the pivot means; and second, by constructing the blades with a bowed profile. Prior art FIGS. 1a-1e show an example of this construction.
FIGS. 1a and 1b show a prior art scissors 101 having a first scissor blade 102 and a second scissor blade 104. Each blade has an inside cutting edge 106, 108 and the blades are pivotally joined at a pivot means 110. The blades pivot one with respect to the other, or both with respect to each other, about pivot means 110 from an open position shown in FIG. 1a to a closed position shown in FIG. 1b. Cutting is effected at the bearing contact of one cutting edge with respect to the other as shown by the circled area 112 in FIG. 1a. As will be appreciated, this bearing contact 112 moves from a point near the pivot means 110 as shown in FIG. 1a along the length of cutting edges 106, 108 to a point 114 shown in FIG. 1b as the blades pivot from the open position shown in FIG. 1a to the closed position shown in FIG. 1b.
In order that the cutting action of the scissors be smooth and consistent throughout the movement of the bearing contact from one position to another the cutting edges 106, 108 must be biased together in a direction parallel to the axis of the pivot means 110. Biasing the cutting edges together when the bearing contact 112 is near the pivot means 10 is usually accomplished by providing the pivot means with a biasing member. For example, pivot means 110 is often threaded with a tightening nut or the like which supplies a biasing force which presses the blades together. In actual practice, most hand operated scissors are relatively loose in the open position and the user manually biases the bearing contact by manipulation of the handles. The biasing force applied at the point of pivot means 110 has little effect, however, in biasing the cutting edges together at the point of bearing contact 114 (FIG. 1b) when the blades are pivoted to the closed position.
In order to effect a sufficient biasing of the blades together at bearing contact 114 and throughout the bearing contacts from 112 to 114, the blades are constructed with a slightly bowed profile. FIG. 1c shows a top view of blades 102 and 104 illustrating their respective bowing towards each other. While it will be generally appreciated how this bowing of the blades serves to bias their cutting edges together as the blades pivot towards the closed position, FIGS. 1d and 1e further illustrate this by showing a top view of the blades when the bearing contact 112 is close to the pivot means (FIG. 1d) and a top view of the blades when the bearing contact 114 is furthest from the pivot means (FIG. 1e).
This conventional construction of a scissors is normally quite acceptable. Generally, the bowing of the blades provides reliable biasing of the cutting edges at bearing contact points distant from the pivot means because the blades are tempered and their bowed profile remains relatively constant throughout the life of the scissors. The primary problem with conventional scissors relates to the biasing of the cutting edges together when the blades are in the open position. The open position biasing relies on the biasing means which is subject to wear and eventual loosening resulting in a failure to bias the cutting edges together when the bearing contact of the cutting edges is at position 112 near the pivot means 110. In addition, the biasing provided by the bowing of the blades when the scissors is in the closed position further operates to loosen the biasing at the pivot means since the biasing at the pivot means continues constantly throughout the movement of the blades from the open to the closed positions. Eventually, the user is forced to compensate greatly for this failure of the scissors, and at some point the scissors is deemed inoperable. An additional problem with conventional scissors is that they often do not provide an even feel over the entire cutting range.
These disadvantages are most problematic in small scissors, and in precision cutting scissors such as surgical scissors. In these types of scissors it is important that the blades move smoothly and evenly throughout the entire cutting range. In addition, the problems of the prior art scissors are magnified in small endoscopic scissor instruments. In a standard type scissors, the operator's hands can reflexively adjust biasing of the blades in the open position by manipulation of the handle to effect a smooth cutting action, and the operator's hands receive feedback as the scissors pivot through their entire cutting range. In an endoscopic instrument, however, the handles of the instrument are not directly connected to the blades. Rather the handles are used to move an actuator means which pivots the blades. Thus, manipulation of the handles to effect biasing in the open position will be completely ineffective.
Various improvements in standard type scissors have been proposed over the years. Several of these proposed improvements involve the use of cammed surfaces. U.S. Pat. No. 1,956,588 to Parker et al. discloses a scissors having two pairs of cammed surfaces in the vicinity of the pivot screw. The scissors disclosed by Parker et al. are "detachable blade scissors" where the blade edges are fine and wire-like and are detachable from the blades. When moving such scissor blades from the wide open position towards a closed position, there is a possibility that the detachable blade edges will cross each other preventing closing of the scissors which is quite the opposite from the problem described above where the cutting edges need to be brought closer together when in the open position. In order to prevent the removable blade edges from crossing, cams are provided on the inner surfaces of the blades so that as the blades are moved from the wide open position towards the closed position, the cams ride up on each other and force the blades apart on the cutting side of the pivot screw in the vicinity where the cutting edges are about to meet. Moreover, since these scissor blades are not bowed, biasing of the cutting edges together throughout the shearing stroke is encouraged by another pair of cammed surfaces on the inside surface of the blades which bias the blades apart behind the pivot screw as the blades are closed.
U.S. Pat. No. 2,627,656 to Richartz also discloses a scissors formed without a bowed profile where each blade is provided with a cam-like bearing face on the inside face of the blade behind the pivot axis. The cams are shaped so that their height increases from their ends to their center so that the high points of the cams are in contact when the blades are in the closed position. These cams take the place of the bowing of the blades and force the blades together as they are closed. Richartz does not provide any means for biasing the cutting edges together when the blades are in the open position. U.S. Pat. No. 4,420,884 to Hembling similarly discloses a non-bowed scissors which are constructed of sheet metal stock and coined in a die assembly to form a control cam which biases the cutting edges of the blades together in place of bowing. Also, U.S. Pat. No. 3,688,402 to Shannon discloses a disposable surgical scissors which have camming surfaces in the vicinity of the pivot screw which bias the cutting edges together as they are moved from the open to the closed position in a manner similar to the scissors disclosed by Richartz and Hembling.
U.S. Pat. No. 4,133,107 to Vogel discloses a scissors with a bearing dimple on the inside face of one blade behind the pivot screw and a mating hole on the other blade. This dimple constantly biases the cutting edges together while the blades are moved from the open position to the closed position and vice versa, but does not bias the cutting edges in the wide open position or in the fully closed position.
U.S. Pat. No. 3,376,641 to Usborne discloses scissors and shears where the inner face of each blade has a shoulder. The shoulders protrude from each face at the same distance as each other from the handle side of the pivot, with the face of each shoulder obliquely transverse of the blade and rising in the direction away from the cutting edge. The transverse obliquity of the shoulders ensures that the inside faces of the blades are mutually oblique behind any point of contact of the cutting edges, thus providing clearance or relief of one blade from the other behind its cutting edge. Usborne teaches controlling the blade positions at all stages between fully open and fully closed by providing the shank of each blade, over a length embracing the shoulder, with a back edge shaped such that progressive contact is made between the two shaped edges, and each such contact is always in a substantially straight line that intersects the pivot axis and the corresponding point of contact between the cutting edges of the blades. Accordingly, as the blades of Usborne's scissors are moved from the open to the closed position, the point of contact between the back edges moves away from the pivot point. While Usborne's scissors seem to provide biasing of the cutting edges together near the pivot point when the blades are open, the geometry of Usborne's shoulders and back edges is complex. Usborne's scissors require that there be a substantial shank on each blade, that is, that there be a significant length of blade behind the pivot point leading to the handle. Moreover, as Usborne's biasing resides specifically on a single moving point or a single line of points, the blades are subject to undue wear on that line of points. Thus, Usborne's scissors are not particularly adaptable for use with endoscopic surgical scissors. The geometry is too complex for such small scissors, and the biasing means requires a substantial shank or tang on each blade behind the pivot point and there is no room for such on the end effector of an endoscopic tool. Even if the geometry could be arranged on such small scissors, with a moving point of contact, the tiny shanks would quickly wear and become unreliable.